Long-Term Outdoor Efficacy Trials of Wood Treated with Organic Biocides and Co-Added Non-Biocidal Additives

Wood preservatives for residential applications are moving towards totally-organic systems. However, organic biocides are relatively expensive and are" biodegraded over the long service life expected from treated wood. To address these two deficiencies we have employed low-cost and benign additives that enhance the efficacy of organic biocides against wood-destroyiog organisms, and/or rednce biodegradlltion by wood-inhabiting organisms such as molda and bacteria. In this p~per we present efficacy results of southern pine sapwood treated with various organic biocides with and without co-added antioxidants, metal chelators, and water repellents and tested for five or more years in ground-contact or above-ground exposure. The addition of a water repellent to above-ground samples greatly enhanced the biocide's efficacy, and the greatest benefit was observed with all three additives. The addition of the antioxidant BHT also reduced biocide depletion in one ground-contact test by about 50"/0. Ground proximity samples treated with relatively low levels of two organic biocides, DCDI and Propiconazole:Tebuconazole, with and without co-added BHT and a metal complexing compound, have been recently installed to verify the above results. INTRODUCTION Wood preservation is undergoing dramatic and rapid world-wide changes. While copper-rich systems are currently employed to treat lumber for residential applications in North America, most professionals expect that totally-organic third generation systems will be required in the future. Twomain concerns with organic biocides are a high cost per unit Weight relative to copper and biodegradationthat can occur over the long service life from various microorganisms, including wood-inhabiting but non wood-destroyiog organisms such as molds and bacteria. To address these problems we are employing non-biocidal additives to increase an organic biocide's efficacy andlor reduce biodegradation (Schnltz and Nicholas 2008). We selected the additives based upon the fundamental knowledge of woOd-degrading mechanisms. Specifically, it is well known that wood-degrading fungi employ metal-mediated reactions (Rodrii\uez et aJ. 2003; Henry 2003) to generate free radicals that attack the wood chemical components (Reading et aJ. 2003; Messner et aJ. 2003; Goodell 2003). Further, it has long been known that many wood extractives have excellent antioxidant and metal complexing properties (Bingnga et aJ. 2005, 2007a and 2007b, 2008; and the terpenoids provide good water repellency that lowers the decay potential. Thus, we have examined the efficacy gained by co-adding antioxidants, metal chelators, and water repellents with various organic biocides to wood (Schultz and Nicholas 2000, 2002, 2005, 2008, Schultz et aJ. 2005). In various laboratory and field trials efficacy increases up to three-fold were often obtained. Furthermore, the addition ofBHT to ground-contact field stakes installed at two locations lowered the depletion of the organic biocide, chlorothalonil, by about 50% after 54 months' exposure (Schultz et aJ. 2006). The purpose of this article is to review above-ground and ground-contact efficacy results of wood treated with various organic biocides, with and without co-added antioxidants" metal complexing compounds, andlor water repellents, and exposed outdoors for at least five years. CHLOROTHALONIL FIELD STAKES WITH BHT Field stakes, 19 x 19 x 457 rom made from defect-free southern pine sapwood, were treated with 0.15, 0.3 and 0.5% chlorothalonil (CTN) dissolved in toluene, with and without the additive BHT at 0, 2 and 4% levels. The stakes were treated by a full cell process then installed in ground contact at the Dorman Lake, MS, and Saucier, MS test sites. After about 3 years the controls had all failed. The results, after 105